Scientists have discovered a new species of giant tortoise in the Galapagos Islands off Ecuador, through genetic testing.

The tortoise is the 15th known species on the archipelago, four of which are extinct. Its discovery was announced in a paper published online by PLOS One.

Yale University biologist Gisella Caccione led the investigation that identified the new species, which lives on Santa Cruz island.

The discovery will help protect and restore the tortoise, which is vulnerable as its numbers are estimated at 250, said Washington Tapia, head of giant tortoise conservation at Galapagos National Park.

That compares with more than 2,000 of the other species living on a different part of the island, Chelonidis porteri.

The new species was christened Chelonoidis donfaustoi after long-time park ranger and conservationist Fausto Llerena.

Mr Tapia said scientists had long suspected that the species was different given that its shell was less dome-like.

The scientists said they suspected Chelonoidis donfaustoi was introduced on Santa Cruz at one point from a different island.

The unique flora and fauna of the Galapagos inspired naturalist Charles Darwin.